The present invention is directed to the combustion of a fuel in a fluidized bed system, particularly a circulating fluidized bed system, and relates to the reactivation of the sorbent material to increase its utilization.
Fluidized bed combustion has gained favor for a number of reasons. An important feature is its ability to burn high-sulfur fuels in an environmentally acceptable manner without the use of flue gas scrubbers. In fluidized bed combustion, much of the sulfur contained in the fuel is removed during combustion by a sorbent material in the fluid bed, usually limestone. Also, in this process, the production of nitrogen oxides is low because of the low temperatures at which the combustion takes place.
One type of fluidized bed combustion is the circulating fluidized bed system. In this system, the gas velocities in the combustor are three to four times as high as in a bubbling fluidized bed system. The particles of sorbent are carried up through and out of the combustor section of the system. The flue gas containing the solid particles is then fed to a separator where the solid particles are separated from the gas by a cyclone. In one arrangement, the solids discharged from the bottom of the cyclone pass through a seal pot, syphon seal or L-valve with a significant portion of the solids going to a solids heat recovery system. The remainder of the solids is reinjected directly back into the combustor. In another arrangement, all solids discharged from the bottom of the cyclone are reinjected directly back into the combustor. In a third arrangement all solids discharged from the bottom of the cyclone are reinjected into the combustor by way of the solids heat recovery system.
In such systems, the limestone is typically fed into the combustor as a separate sorbent feed. In the process, the limestone is calcined to form calcium oxide, CaO, which then reacts in the combustor with the oxides of sulfur forming calcium sulfate, CaSO.sub.4. Both the CaO and the CaSO.sub.4 are in solid form at the operating conditions of a fluidized bed. Since the sulfur oxides react with the CaO on the surface of the sorbent particles, the end result is solid particles with a core of unreacted CaO and an outer layer of CaSO.sub.4. The unreacted CaO core is prevented from reacting due to the outer surface layer of CaSO.sub.4 because the gaseous sulfur oxides cannot effectively penetrate to the core. The consequence is the requirement of an excess of limestone over what would be stoichiometrically required for any given level of sulfur removal and the under utilization of the limestone. This process also can be used within a bubbling fluidized bed.